1. Field of the Invention
This invention relates to electric circuits, and in particular, to an optically isolated control circuit which allows continuous monitoring of the circuit being controlled to verify its proper operation.
2. Description of the Prior Art
In the design of many control systems it is frequently desirable to situate the object or device being controlled remotely from the control input signal. Furthermore, in many such installations, for example, nuclear power plants, it is desirable to completely isolate the control device from the control input. Although this has been achieved with prior art control systems, such control systems have typically been unable to monitor the correct operation of the device or object being controlled. For example, in prior art control systems, the control input signal has been supplied to a remotely situated object being controlled using optical or electronic techniques, but the station from which the control input signal was generated has been unable to detect the correct functioning of the isolated controlled system.
Testable optically coupled solid state relay switches are intended to operate continuously and reliably with the greatest possible immunity from their environment, which is a nuclear power plant. The relays are physically shielded by a concrete wall having glass or quartz windows through which light signals optically couple the relay to a controller. The prior art coupling was deficient in that the relay input was responsive to different kinds of light including contamination signals. For example, the relay would not be properly isolated from a fire in the controller on the other side of the wall if light from the fire penetrated the window.
In the normal operation of the reactor, the switch elements continuously conduct a large current to solenoids holding spring loaded valves closed. The continuous current generates heat in the switches, and every degree of heat hastens switch deterioration. Heat dissipation in the switches should be minimized as much as possible.
The switching elements used in the relay should have a fast response time so that the relay can be accurately tested. Using a conventional full-wave rectifying diode-bridge to supply voltage to the switching elements provides a fast diode pump turn-on time, but turn-off time is delayed while the capacitance in the switching elements discharges. A small enough bleed resistor to rapidly discharge this capacitance would dissipate too much heat. Therefore, an improved fast turn-off circuit for voltage supplied by a diode bridge is needed.
Finally, the relay should have high sensitivity to failure of the switch to open. Verification of output contact current switching must preserve the isolation of the contact current. The prior art method connected the output line to a resistor shunt, across which the voltage drop was measured to determine whether current was flowing. The resistor shunt violated the isolation of the contact current.